<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
</head>

<body>
    <div id='app'>
        <div>
            <button @click="sub">-</button>
            <span>{{ num }}</span>
            <button @click="add">+</button>
        </div>
    </div>
    <script>
        var app = new Vue({
            el: '#app',
            data: {
                num: 1
            },
            methods: {
                add: function () {
                    if (this.num < 10) {
                        this.num++;
                    } else {
                        alert("到头啦")
                    }
                },
                sub: function () {
                    if (this.num > 0) {
                        this.num--;
                    } else {
                        alert("到底啦")
                    }
                },
            }
        })
    </script>
</body>

</html>